The orienting layer is of particular significance in (electro-optical) liquid crystal devices. It serves the purpose of guaranteeing a uniform and disturbance-free alignment of the molecular axes.
Uniaxially rubbed polymer orienting layers, such as e.g. polyimide, are usually used for the orientation of liquid crystal molecules in liquid crystal indicators (LCD's). The direction of rubbing provides the orienting direction in this procedure. However, some serious disadvantages are associated with the rubbing and these can severely influence the optical quality of liquid crystal indicators. Thus, by rubbing, dust is produced which can lead to optical failure in the display. At the same time, the polymer layer becomes electrostatically charged, which, for example, in the case of Thin Film Transistor (TFT)-TN LCD's can lead to the destruction of the thin layer transistors which lie below. For these reasons the yield of optically faultless displays in LCD production has hitherto not been optimal.
A further disadvantage of rubbing is that it is not possible to produce in a simple manner structured orienting layers, since the direction of orientation can not be varied locally by rubbing. Accordingly, mainly uniformly directed layers of large area can be produced by rubbing. Structured orienting layers are, however, of great interest in many areas of display technology and integrated optics. For example, the viewing angle dependence of Twisted Nematic (TN) LCD's can be improved with them.
Orienting layers in which the orienting direction can be produced by irradiation with polarized light have been known for some time. Thereby, the problems inherent in rubbing can be avoided. In addition, there exists the possibility of producing a different orienting direction in a regional manner and thus to structure the orienting layer.
One possibility for the structured orienting of liquid crystals utilises the isomerizing capability of certain dye molecules in order to photochemically induce a preferred direction by irradiation with polarized light of suitable wavelength. This is achieved, for example, by admixing a dye with an orienting polymer and then irradiating with polarized light. Such a guest/host system is described, for example, in U.S. Pat. No. 4,974,941. In this system azobenzenes are incorporated in polyimide orienting layers and subsequently irradiated with polarized light. Liquid crystals, which are in contact with the surface of a thus-irradiated layer, are orientated correspondingly to this preferred direction. This orienting procedure is reversible, i.e. by repeated irradiation of the layer with light of a second polarization direction the already inscribed direction of orientation is again reversed. Since this re-orientation procedure can arbitrarily be repeated frequently, orienting layers on this basis are less suitable for use in LCD's.
A further possibility for the production of a high resolution orienting pattern in liquid crystal layers is described in Jpn. J. Appl. Phys. Vol. 31 (1992), 2155. In this procedure the dimerization of polymer-bonded photoreactive cinnamic acid groups induced by irradiation with linear polarized light is used for the structured orienting of liquid crystals. In contrast to the above-described reversible orienting procedure, an anisotropic polymer network is produced in the photo-structurable orienting layers described in Jpn. J. Appl. Phys. Vol. 31 (1992), 2155. These photo-orientable polymer networks are primarily usable where structured or non-structured liquid crystal orienting layers are required. Apart from use in LCD's, such orienting layers can also be used, for example, for the production of so-called hybrid layers as are exemplified in European Patent Applications EP-A-0 611 981, EP-A-0 689 084, EP-A-0 689 065 and EP-A-0 753 785. Optical elements such as, for example, non-absorptive color filters, linear and circular polarizers, optical delay layers, etc. can be realised using these hybrid layers of photo-structured orienting polymers and cross-linkable lower molecular liquid crystals.
Cinnamic acid polymers, which are suitable in principle for the production of such anisotropic cross-linked, photo-structured orienting layers for liquid crystals, are described in EP-A-611,786. These cross-linkable cinnamic acid derivatives are basically linked via the carboxyl function of the cinnamic acid (phenylacrylic acid) and a spacer to the main chain of the polymer. In these polymers the dimerizable acrylic ester group of the cinnamic acid is always aligned "inwards" to the spacer or polymer backbone, while the aromatic residue is always oriented "outwards" from the polymer backbone.
It has now been found that this type of cinnamic acid alignment in known photopolymers is by no means optimal. Concurrent photochemical reactions disturb the orienting capacity. The known cinnamic acid polymers have an inadequate photochemical long-term stability. For example, a lengthy UV light irradiation of a pre-produced orienting layer leads to the destruction of the originally present orientation. Multiple irradiations, in which an already present orienting layer having a predetermined inscribed pattern is irradiated once more in order to orientate the still non-irradiated regions in another direction, can only be carried out when the previously irradiated sites are covered by a mask. On the other hand, the already oriented regions of the layer can lose their structure wholly or in part by photochemical side-reactions.
A further disadvantage of the previously used cinnamic acid polymers is that no tilt angle occurs in the case of orienting surfaces produced from these materials by a simple irradiation with polarized light. In particular, for use in LCD's the orienting layer must produce a tilt angle in addition to the orienting direction.
In the case of the aforementioned uniaxially rubbed polymer orienting layers this tilt angle is already produced by the rubbing procedure on the polymer surface. When a liquid crystal is brought into contact with such a surface, then the liquid crystal molecules lie not parallel, but inclined to the surface, with the tilt angle thus being conferred to the liquid crystal. The size of the tilt angle is thus determined not only by rubbing parameters such as, for example, feed velocity and contact pressure, but also by the chemical structure of the polymer. A tilt angle between 1.degree. and 15.degree. is required for the production of liquid crystal indicators depending on type. The largest tilt angles are required especially for Supertwisted Nematic (STN) LCD's in order to avoid the occurrence of so-called fingerprint textures. In TN and TFT-TN LCD's the direction of rotation and the direction of tilt is defined by the tilt angle, whereby "reverse twist" and "reverse tilt" phenomena are prevented. While reverse twist in the unswitched state gives rise to fields having a false sense of direction, which is noticeable optically in a speckled appearance of the indicator, reverse tilt is noticeable optically to a very disturbing extent primarily upon switching the LCD's by tilting the liquid crystal in different directions. Reverse twist can be prevented by doping the liquid crystal mixture with a chiral dopant having a suitable direction of rotation. However, in order to suppress reverse tilt there has hitherto been no alternative possibility to the use of orienting layers having a tilt angle.
Cinnamic acid esters which are not, as described above, bonded to a polymer backbone, but which are linked via the spacer with a trialkoxysilane group have recently been reported in Liq. Cryst. 20, 171 (1996). In this case, the trialkoxysilane group serves to anchor the cinnamic acid unit to the substrate as a carrier, for example to glass. The spacer, which links the trialkoxysilane group with the cinnamic acid ester, is thereby always situated in the 2-position (ortho-position) of the cinnamic acid ester. For the production of the orienting layer, the trialkoxysilanes are firstly applied from a solution to the glass carrier. Thereafter, the orientation is effected by irradiation with linear polarized light of 259 nm wavelength. The capacity of the thus-prepared layer to orient liquid crystals is ascribed to a reversible Z/E isomerization. On the other hand, when the cinnamic acid molecules are irradiated at 330 nm, they become cross-linked. Thereby, the orientation capacity is lost in proportion to the degree of cross-linkage.
The orienting layers produced in this manner have the same disadvantages which the cinnamic acid polymers described above have. Also, they have an insufficient photochemical and thermal stability, since the Z/E isomerization is reversible and therefore leads to problems of re-orientation in the case of multiple irradiation. Moreover, they do not also have the capability of inducing a tilt angle.